This invention involves the field of nonlinear optics and is concerned with optical phase-conjugate communication systems utilizing mutually incoherent laser beams.
On a fundamental level, the phenomena of nonlinear optics arise out of the interaction of light and matter. This interaction is nonlinear for those materials in which incident light can change the material's index of refraction, thereby affecting the frequency, intensity, and/or phase of the light as it propagates through the material. By providing a means to manipulate these properties of a coherent beam of light, nonlinear optics has made possible new optical applications in such fields as optical information processing, optical computing, laser beam control, and optical sensors.
The branch of nonlinear optics known as phase-conjugate optics deals with the generation, propagation, and application of phase-conjugated beams of light. If a light beam is considered as the motion of a family of wavefronts in space, the phase-conjugate of that light wave has exactly the same set of wavefronts as the initial wave, but the phase-conjugate wave moves in the opposite direction. Consequently, a phase-conjugate beam can be considered a time-reversed replica of an incident beam, capable of retracing the path of the incident beam. A device which can generate such a beam is known as a phase-conjugate mirror.
One important way in which phase-conjugated light can be produced is by employing the photorefractive effect, a nonlinear optical phenomenon which occurs in photorefractive crystals, such as barium titanate (BaTiO.sub.3) and strontium barium niobate (Sr.sub.l-x Ba.sub.x Nb.sub.2 O.sub.6). When a photorefractive crystal is illuminated with two mutually coherent laser beams, interference between the two beams causes an optical fringe pattern to be formed within the crystal. The fringe pattern induces a separation of electrical charges within the material. This charge separation creates an electric field that, in turn, induces a local variation in the index of refraction of the crystal via the Pockels effect, resulting in a volume index grating that allows the exchange of energy between the beams. Phase-conjugate light is produced by a readout beam of the same frequency, counterpropagating to one mutually coherent write beam which diffracts off the volume hologram (index grating) in a direction counterpropagating to the other write beam.
A distinctive feature of the exchange of energy by means of photorefractive two-beam coupling is the lack of any phase crosstalk in the process, i.e., one beam can be amplified at the expense of the other without the aberrations and frequency differences of the donor beam being transferred to the acceptor beam. The discovery of this phenomenon has led to a variety of new applications, including beam processing techniques, such as image amplification, laser beam cleanup, and beam combining, as well as device structures such as ring oscillators, laser radars, and sensor protection devices. In photorefractive crystals with a large two-beam coupling gain, a stimulated effect known as beam fanning can occur. In this effect, scattered light generated from scattering centers (defects or impurities) in the crystal can be amplified via two-beam coupling. The end result is that an initially well-defined incident laser beam spreads out spatially upon exiting the crystal. Beam fanning is a crucial part of many photorefractive device applications including those described here.
Mutual coherence of the two beams writing the photorefractive hologram is required for both photorefractive two-beam coupling and photorefractive phase conjugation. This mutual coherence requirement, however, severely limits the practicality of the photorefractive effect, particularly for phase conjugation over large distances. In U.S. Pat. Application No. 228,437, now U.S. Pat. No. 4,911,537 ("Bird-Wing Phase Conjugator using Mutually Incoherent Laser Beams", filed Aug. 5, 1988, Mark D. Ewbank, applicant) a new phase conjugation technique is disclosed which provides for efficient phase conjugation using mutually incoherent beams. This new technique would be particularly useful in communication systems which use mutually incoherent beams for laser sources separated by large distances.